A storm system that dropped a foot of snow in the Rockies was making travel hazardous as it headed east on Wednesday, menacing the Plains with heavy snow and threatening turbulent weather -- even tornadoes -- in parts of the Midwest.

The system was expected to affect more than 36.5 million people from Colorado through Ohio and from Texas north through Michigan. The National Weather Service issued tornado watches for most of the day in parts of Nebraska, Kansas, Missouri and Iowa.

The storm left Denver covered in 5 inches of snow on Wednesday, and Colorado highway patrol troopers reported several crashes north of the city on windy, icy Interstate 25 near the Wyoming border. Wyoming authorities also shut down large sections of roadways.

The Nebraska State Patrol reported several minor accidents involving vehicles sliding off icy or slushy roadways, but no injuries have been reported.

Alan Salyards said truckers coming through his Flying J Travel Plaza near Big Springs, a Nebraska town along the Colorado boarder, didn't seem especially concerned. He said plow trucks were working all morning to keep the roads passable.

"The drivers are just doing their normal thing," he said.

At Denver International Airport, spokesman Heath Montgomery said airlines canceled about 50 flights in anticipation of the bad weather -- out of the airport's 1,500 daily flights -- and ground crews kept up with the snowfall. No major delays were reported.

The back side of the fast-moving system will run into more cold air over Kansas and Nebraska, leaving behind up to 8 inches of snow in some spots, said Jared Guyer, a forecaster with the National Weather Service's Storm Prediction Center.

As the system spreads east, its southern portion will run into warmer, humid air and raise the potential for severe thunderstorms and even tornadoes, he said.

A winter storm warning or blizzard warning was in effect through Wednesday for parts of Utah, New Mexico, Wyoming, Colorado, Nebraska and Kansas. Sustained winds of 30 to 40 mph were expected, with gust up to 55 or even 60 mph.

The weather service cited the potential for white-out conditions and urged people to stay home.

At Pi Kappa Cino Coffee in Sterling, a town in far northeast Colorado, workers were checking their heaters and stocking up on coffee Tuesday to handle the first significant snowfall of the season.